Tilapia is an extremely mild fish. So mild, it almost lacks its own character. Consequently, the fish is quite often served with distinctive seasoning or saucing. In any case, whatever you use as the principle seasoning will stand out more than it might with another, more assertive fish. Assuming you're going with a parmesan type cheese crust, which is not exactly what you'd call bold, your sides should be chosen accordingly. They should be neither so assertive as to bury the fish, nor so bland the dish lacks interest. Also, baked fish is usually pretty dry.
Ratatouille is a good choice. However, let me suggest potatoes sauteed with rosemary, and broccoli with balsamic vinegar. Both of these are so simple they won't upstage the fish, their tastes are strong and straightforward, and they bring moisture and fat (in the good way). Also, the acid in the balsamic will perk things up without competing with the lemon most people serve with fish.
Cut the stems from the broccoli. Trim the stems by removing and reserving the leaves. Peel the stems with a vegetable peeler, and cut the stems into thin coins. Divide the crowns into into "florets".
Peel the potatoes or not, as you prefer. Cut them in bite sized dice, put in a pan with salted water, bring to the boil, and remove from the heat. Drain the potatoes well. Clean the pan, and put about an inch of water in the bottom and add a little salt, and set aside.
Preheat a skillet large enough to hold the potatoes in a single layer over a medium-high burner. When the skillet is hot, add enough extra virgin olive oil to saute. When the oil has come to temp (it seems to shimmer and runs like water in the pan), add the potatoes. Allow the potatoes to cook undisturbed for a few minutes until they become fragrant and start to brown.
While the potatoes are starting to brown, bring the water you set aside to the boil. When the water boils, add the broccoli, cover, and reduce heat to a simmer. Allow the broccoli to simmer for about five minutes, and remove from the flame, still covered.
By this time the potatoes will have begun to brown. Use a spatula to release them from the pan and turn them. Add more oil if necessary. Again, allow to brown more or less undisturbed for a few minutes. Again, release them from the pan with a spatula. After they are released, they will not stick so easily, and you can begin to saute(later) in the usual way, tossing frequently.
Drain the broccoli, and return it to the pan with a little butter and a little salt. Allow the butter to melt with residual heat. Salt lightly.
Toss the potatoes and check for even coloring. When the potatoes start to become golden add a little finely chopped fresh rosemary (a little goes a long way) and plenty of salt and pepper. Continue tossing every now and then until GBD (golden brown and delicious).
Splash some balsamic generously over the broccoli, toss and serve immediately.